Braking energy converters are known by the general term "retarder". Some retarders operate according to the hydrodynamic turbine-pump principle with a built-in cooling water oil cooler. Their design resembles that of a hydrodynamic coupling, in which a stationary impeller stands opposite a rotating impeller connected with the propeller shaft of the vehicle. In this stator, the kinetic energy imparted to the hydraulic oil by the centrifugal force from the rotation of the rotor is reduced and eliminated or converted into heat. Also known in the art are electromagnetic eddy current brakes, which necessarily have a very heavy design, and whose required fan disks constantly consume power. Finally, there are hydrodynamic torque converters, which are arranged in front of the power shift gear in vehicles, and were modified to function as retarders of the kind described above using complex technology.
The known retarders have relatively large dimensions, not least because the braking capacity may only be raised by increasing the speed, oil volume and diameter of the two impellers, since the centrifugal force can only go up at a higher radius.